1. Field of the Invention
The present invention relates to a polyimide precursor, a resin composition comprising the polyimide precursor, a pattern forming method using the resin composition, and articles produced by using the resin composition.
2. Description of the Related Art
Polyimides are polymers synthesized from diamines and acid dianhydrides. Diamines react with acid dianhydrides in solvent to produce polyamic acids, which are precursors of polyimides and give polyimides through dehydration and ring-closure reaction. In general, since polyimides have poor solubility in solvents and are difficult to process, they are often obtained by making their precursors, which are polyamic acids, into a desired form followed by heating. However, polyimide precursors are often sensitive to heat or water and thus inferior in storage stability (J. A. Kreuz, “J Polym. Sci. Part A: Polym. Chem.” vol. 28, 1990, p. 3787). It is known that polyimide precursors (polyamic acids) are generally liable to be hydrolyzed at room temperature, which leads a decrease in molecular weight. This is said to result from the fact that the polyaddition reaction for preparing polyamic acids is an equilibrium reaction. More specifically, in a polyamic acid, cleavage of amino bonds into acid anhydrides and amino groups and their recombination are continuously repeated. Acid anhydride groups thus contained in the system react with moisture in the same system to give a dicarboxylic acid, so that the acid anhydride groups are removed from said equilibrium reaction system, which directs the equilibrium in the direction to cut the amide bonds (in the direction to decrease the molecular weight of the polyamic acid). An improved polyimide was developed in consideration of this point, in which a skeleton excellent in solubility is introduced to a molecular structure thereof so that the polyimide can be easily dissolved in a solvent and formed or applied. However, such a polyimide tends to be inferior in chemical resistance or adhesion to a substrate to the polyimide obtained by the method using a precursor.
Further, a polyimide precursor has been proposed in which carboxyl groups of a polyamic acid are esterified in terms of storage stability (Japanese Patent Application Laid-Open (JP-A) No. Sho. 61-293204). However, this polyamic ester is disadvantageous in that it needs a two-step reaction, purification for removing a condensing agent, and high production costs. In addition, ester bonds are hard to be thermally decomposed; therefore, even after the polyimide precursor is imidized by heat treatment at 300° C. or more to give a polyimide, a thermal decomposition residue derived from ester portions remains in the polyimide, which is responsible for deterioration of properties of the polyimide such as linear thermal expansion coefficient and humidity expansion coefficient.
JP-A Nos. 2002-121382 and 2001-194784 have proposed negative-working photosensitive polyimides produced by the combination of a photoradical generator and a reaction product of a vinyl ether containing photoreactive groups with a polyamic acid. In these inventions, however, since a pattern is formed by cross-linking reaction, the molecular weight of photoreactive portions is increased and decomposed products of the photoreactive portions are likely to remain after imidization. Therefore, these inventions are disadvantageous in that components other than the polyimide remain in a film after imidization, which are responsible for outgassing, etc. Furthermore, they are still disadvantageous in the following respects: gelation is likely to occur upon synthesis and/or during storage because the polyimide contains large amounts of cross-linking components; and portions derived from the vinyl ether is eliminated from the polyamic acid over time, which leads to a decrease in pattern forming ability, because a photoradical generator and a compound containing amino groups, which is a sensitizer of the photoradical generator, are added to the polyamic acid.
JP-A No. 2005-115249 has disclosed a resin composition containing a polyimide precursor in which phenolic hydroxyl groups are each introduced into a carboxyl group of a polyamic acid by an ester bond, a divinyl ether compound having two vinyl ether portions and a thermally decomposable linking group, and a photoacid generator. In the process of forming a film of the resin composition, heat applied to form a film causes a reaction between the phenolic hydroxyl groups and vinyl ethers to form acetal bonds, thereby cross-linking the polyimide precursor. Then, the film is exposed to generate an acid, heated to break the acetal bonds in the exposed portion only, and developed with a basic aqueous solution, thereby obtaining a positive pattern. However, JP-A No. 2005-115249 is disadvantageous in the following respects: it is necessary to add a polymer having a plurality of vinyl ether groups as a cross-linking agent, in a proportion of about 25% based on the whole polyimide precursor; a complicated synthesis process is necessary to introduce phenolic hydroxyl groups into a polyamic acid; and the polyimide precursor is reacted with a vinyl ether to form a film, so that the shape of a pattern, etc., is significantly affected by film forming conditions or the like and thus is susceptible to changes in the process.